Nightmares are dreams with a disturbing content. I like to distinguish the following categories:
1. Nightmares that are frightening during the dream.
2. Violent dreams, however without the expected level of pain or fear.
3. Dreams that become nightmares after waking up.
The first category is very common and often involves chases by some dark unseen enemy. The advice in popular dreaming literature is to realize to confront your enemy in your dreams. More on this follows in a next chapter.
The second category is more disturbing than frightening. Violence in dreams can symbolize psychological problems or the resolving of a particular problem, but when you're ill it is just as likely to represent the physical problems you're dealing with. For example, I see my own dreams about being stabbed in the back as a symbol for how I feel betrayed by suddenly getting a disease. There does not seem to be any reason for this disease and there is no known cause or explanation. But most of my violence dreams show a direct relationship to my physical condition. The more violence, the higher the need for me to get some drastic rest immediately.
I have no idea how common the third category is. I'm thinking here about dreams that suddenly make you realize things about yourself that aren't always flattering, or dreams that show you possible scenarios of future events you weren't prepared for.